As finer design is applied to semiconductor devices, there is an increasing need for developing finer processing with photolithography technique used for semiconductor device manufacturing processes. The exposure light for photolithography was mainly ArF excimer laser light of 193 nm around the year of 2000. However, the current trend is to apply light called extreme ultraviolet light (hereinafter referred to as EUV light) with a wavelength range of mainly 15 nm or less, particularly of 13.5 nm.
EUV light is very easily absorbed by nature by most substances. Therefore, instead of the transmissive photomasks based on conventional art, reflective photomasks are used for exposing EUV light (hereinafter referred to as EUV masks). Such an EUV mask is configured by, for example, forming a multilayer reflection film that includes a molybdenum (Mo) layer and a silicon (Si) layer, which are alternately laminated on a glass substrate, and forming a light absorption film having tantalum (Ta) as a main component on the multilayer reflection film, followed by forming a circuit pattern on the light absorption film.
Generally, in lithography using a reflective EUV mask, EUV light is made incident on the EUV mask at an angle of approximately six degrees, and the reflected EUV light is led to a wafer that is an object of exposure to expose an EUV light-sensitive resist formed on the wafer.
Thus, in performing lithography mentioned above, the optical axis of the EUV light incident on the EUV mask is inclined and the EUV light is reflected by the circuit pattern on the EUV mask. However, it is pointed out that, depending on the direction of the reflected light, a shadow is cast on a part of the light absorption film, causing a phenomenon of hindering irradiation of the EUV light onto the wafer (a projection effect). To minimize the projection effect, there is a method used for reducing the influence of the shadow by reducing the thickness of the light absorption film in which a circuit pattern is formed.
However, simply reducing the thickness of the light absorption layer leads to insufficiency in the light attenuation which the light absorption layer is originally required to exert. Therefore, there is a concern that the reflection of the EUV light irradiated to the resist on the wafer will increase more than necessary and thus the accuracy of forming a circuit pattern is degraded. In addition, in an actual exposure process, chips are often mounted on a single wafer in a multifaceted manner. Therefore, there is particularly a concern that the exposure of the resist increases in a boundary region where chips are adjacent to each other. Specifically, multiple exposures are unavoidably caused in the boundary region between the chips to adversely affect the accuracy of forming a circuit pattern. The multiple exposures will result in decreasing the quality and the throughput of chips obtained at a later stage. Therefore, when the light absorption layer is made thinner, a method of forming a groove is used. With this method, the EUV mask is grooved by removing the light absorption film and further removing the multilayer reflection film so as to reach the surface of the glass substrate surface (see PTL 1). This method is used for the purpose of using the groove as a shading region having high shading properties for the wavelengths of the EUV light, minimizing the reflection of the EUV light in the shading region, and minimizing multiple exposures in the boundary region between adjacent chips.
However, in many cases, light sources that generate EUV light emit not only light of the EUV range of about 13.5 nm but also light with a wavelength band covering from a vacuum ultraviolet range (about 140 nm) to a near infrared range (about 800 nm). This wavelength band is generally called out-of-band. Thus, on an EUV mask, light with the out-of-band wavelength (hereinafter referred to as out-of-band light) is also incident together with the EUV light. In the shading region of the EUV mask, the shading properties for EUV light are relatively high, whereas the shading properties for out-of-band light are relatively low. In the shading region, of the light emitted from a light source, reflection of the EUV light can be mostly prevented, however, part of the out-of-band light is reflected in the shading region and irradiated onto the wafer. This raises a problem of causing multiple exposures in the boundary region between the chips.
PTL 2 discloses a technique that can solve such a problem. According to this technique, a finely structured pattern is formed on a rear surface of a glass substrate, which is a surface opposite to the multilayer reflection film. By forming the pattern, after out-of-band light incident on a shading region has reached the rear surface of the glass substrate from vacuum, the reflection of the light on a rear surface conductive film is minimized.